Low nox, high efficiency, high temperature, staged recirculating burner and radiant tube combustion system

ABSTRACT

Embodiments of the present invention include high-temperature staged recirculating burners and radiant tube burner assemblies that provide high efficiency, low NOx and CO emissions, and uniform temperature characteristics. One such staged recirculating burner includes a combustion tube having inside and outside helical fins forming opposing spiral pathways for combustion gases and products of combustion, a combustion nozzle coupled to the combustion tube, a gas tube running axially into the combustion tube, and a staging gas nozzle coupled to the gas tube, where the staging gas nozzle includes radial exit holes into the combustion tube and an axial gas staging tube extending into the combustion nozzle to stage combustion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 16/457,774, entitled “LOW NOX,HIGH EFFICIENCY, HIGH TEMPERATURE, STAGED RECIRCULATING BURNER ANDRADIANT TUBE COMBUSTION SYSTEM,” by Chris Edward VANDEGRIFT et al.,filed Jun. 28, 2019, which is a continuation application of and claimspriority under 35 U.S.C. § 120 to U.S. patent application Ser. No.14/863,563, entitled “LOW NOX, HIGH EFFICIENCY, HIGH TEMPERATURE, STAGEDRECIRCULATING BURNER AND RADIANT TUBE COMBUSTION SYSTEM,” by ChrisEdward VANDEGRIFT et al., filed Sep. 24, 2015, now U.S. Pat. No.10,458,646, which claims priority under 35 U.S.C. § 119(e) to U.S.Patent Application No. 62/055,095, entitled “LOW NOX, HIGH EFFICIENCY,HIGH TEMPERATURE, STAGED RECIRCULATING BURNER AND RADIANT TUBECOMBUSTION SYSTEM,” by Chris Edward VANDEGRIFT et al., filed Sep. 25,2014, all of which are assigned to the current assignee hereof andincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Combustion of fossil fuels introduces emissions into the atmosphere,such as nitrogen oxides (NOx). NOx emissions arise from nitrogen presentin the combustion air and from fuel-bound nitrogen in coal or fuel oil,for example. Conversion of fuel-bound nitrogen to NOx depends on theamount and reactivity of the nitrogen compounds in the fuel and theamount of oxygen in the combustion area. Conversion of atmosphericnitrogen, N2, present in the combustion air to NOx istemperature-dependent; the greater the flame temperature in thecombustion area, the greater the resultant NOx content in the emissions.One way of reducing NOx content is to create a fuel-rich combustion areafollowed by a fuel-lean combustion area, which can be achieved bystaging the introduction of air into the combustion chamber.Recirculating flue gas into the flame is another technique to limit NOxemissions.

SUMMARY OF THE INVENTION

Embodiments of the present invention include high-temperature stagedrecirculating burners and radiant tube burner assemblies that providehigh efficiency, low NOx and CO emissions, and uniform temperaturecharacteristics. One such staged recirculating burner includes acombustion tube having inside and outside helical fins forming opposingspiral pathways for combustion gases and products of combustion, acombustion nozzle coupled to the combustion tube, a gas tube runningaxially into the combustion tube, and a staging gas nozzle coupled tothe gas tube, where the staging gas nozzle includes radial exit holesinto the combustion tube and an axial gas staging tube extending intothe combustion nozzle to stage combustion.

Some embodiments of such a staged recirculating burner can include aceramic wall as part of the combustion tube that separates the flow ofthe combustion gases and the flow of the products of combustion, wherethe directions of flow of the combustion gases and the products ofcombustion are opposite. In many embodiments, the combustion tube can bemade of silicon carbide, and/or the combustion nozzle is a conicallyshaped combustion nozzle.

A staged recirculating burner can further include a heat exchangercoupled to the combustion tube that heats the combustion gases providedto the combustion tube using the products of combustion from thecombustion tube. In such embodiments, the combustion tube and the heatexchanger may be connected by a specialized silicon carbide threadallowing the combustion tube to be adjustable. The gas tube of theburner may extend through the central axis of the heat exchanger andinto the combustion tube.

In some embodiments, the staging gas nozzle injects gas radially into aspiral flow of preheated air flowing through the combustion tube and, insuch embodiments, the staging gas nozzle can inject only a portion ofthe gas through the radial holes of the staging gas nozzle, thuscreating a gas mixture that is substantially lean to suppress thetemperature of products of combustion, and inject the reminder of thegas through the axial gas staging tube.

An example radiant tube burner assembly includes aa staged recirculatingburner, as described above, an outer radiant tube coupled to the burner,an inner recirculating tube located concentrically inside the outerradiant tube, where the outer radiant tube and the inner recirculatingtube forming an annulus between the outer and inner tubes, and a turningvane spacer located inside the outer radiant tube and positioned betweenthe distal end of the inner recirculating tube and the distal end of theouter radiant tube to cause products of combustion to flow through theannulus between the outer radiant tube and the inner recirculating tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic diagram illustrating a single-ended radiant (SER)tube burner assembly, according to an example embodiment of the presentinvention.

FIG. 2 is a schematic diagram illustrating a burner assembly, accordingto an example embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating a combustion air heatexchanger inlet, according to an example embodiment of the presentinvention.

FIG. 4 is a schematic diagram illustrating a heat exchanging surface,according to an example embodiment of the present invention.

FIG. 5 is a schematic diagram illustrating a gas tube/gas nozzleassembly, according to an example embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

Structure of Staged Recirculating Burner and Radiant Tube Burner System

An example embodiment of a self-recuperative, single-ended, radiant tubeburner system assembled within a chamber of a conventional heat treatingfurnace is shown in FIG. 1. One wall 20 of the furnace is shown in FIG.1 and is typically made of a refractory material 21 whose outer side iscovered by a metal skin 22. The self-recuperative single-ended radianttube burner system includes an elongated (outer) radiant tube 23disposed within the furnace chamber and made of a silicon carbide,metallic, or other suitable heat-resistant material. The outer radianttube 23 extends through a cavity 25 in the furnace and the downstreamend of the outer tube is closed as indicated at 24. The outer radianttube 23 includes an outer flange 26 that is secured into a furnacemounting flange 14 on the outside wall of the furnace and may be securedinto position using an exhaust housing flange 28 that mounts a burnerassembly 30 to the furnace. The burner assembly 30 is secured to and ispartially disposed in the radiant tube 23 to generate a high velocity,high-temperature flame to appropriately heat the furnace. Assembledconcentrically inside the outer radiant tube 23 is an inner radiant tube27 made of silicon carbide. The inner radiant tube 27 is properlypositioned away (e.g., three inches) from the downstream (distal) end ofouter tube 23 using spacer (turning vane) 29. The length of the innertube 27 is furnace specific but the inlet face 31 is aligned coincidentto the inside furnace refractory wall 20. Likewise, the outlet componentof burner assembly 30 (i.e., combustion nozzle 32) is also parallel tothe inside wall of the refractory 20 and the face of the inner radianttube 31.

Referring to FIG. 2, there may be a number of components, for example,that comprise an example staged recirculating burner assembly 30: aninlet housing 34, gas tube 37, exhaust housing 43, heat exchanger 42(such as, for example, a heat exchanger as disclosed in U.S. Pat. No.8,162,040), gas nozzle 51, and combustion tube 47. Combustion air isrouted via a pipe into the burner inlet housing 34 via aperture 36, andcommunicates with a blower (not shown) or other means for producing aflow of forced combustion air. Also connected to the burner inlethousing 34 is a fuel supply line 35 that is in communication with anelongated gas pipe 37.

The gas pipe 37 extends through the central axis of the assemblydownstream through heat exchanger 42 and into the combustion tube 47where it supports gas nozzle 51. The inlet housing 34 and gas nozzle 51may be designed specifically to communicate in such a way that spark rod19 and flame sensor 18 (FIG. 1) can be placed inside of the gas tube 37.As shown in FIG. 5, the spark rod 19 may extend through gas nozzle 51via 51B and an electrode can be placed approximately an inch downstreamSIB in order to ignite the fuel/air mixture discharged therefrom. Flamesensor 18, connected to an indicator, may extend through gas nozzle 51via 51C and can be positioned approximately three inches downstream.Flame sensor 18 detects the presence of a flame and, via appropriateindication, when the flame has been extinguished. Gas tube 37 may bemade from 1.5″ schedule 80 stainless steel tube in order to withstandthe exposure to the high temperatures generated by the combustion itselfjust downstream and the recuperation of the combustion gases by both thecombustion tube 47 and the heat exchanger 42. Gas nozzle 51 may be madeof silicon carbide to provide enhanced exposure to the high-temperatureenvironment and allow steady and consistent delivery of natural gas. Gasnozzle 51 may be assembled into gas tube 37 and fixed using set screws(e.g., three set screws 120 degrees apart 37A), as shown in FIG. 3. Asmall gap 37B created by the inner diameter of the gas tube 37 and outerdiameter of the gas nozzle 51 may be sealed off using ceramic putty,ensuring no gas flows through the gap space.

Once combustion air has entered the inlet housing 34 via inlet port 36,it fills void 38 (FIG. 2) around the gas tube 37. A gasket 39 betweenthe burner inlet housing 34 and the heat exchanger spacer flange 40seals the combustion air into this void and forces the air to pass intothe introductory port section (e.g., about three inches long) of theheat exchanger 42 where the air begins to wind into individual portsthat form into rounded rectangular channels. In some exampleembodiments, there may be six individual ports. FIG. 3 illustratesexample inlet ports of the heat exchanger 42.

The heat exchanger 42 may be held in place via compression of a spacerflange 40 between the burner inlet housing 34 and exhaust housing 43(e.g., the inlet into which the heat exchanger flange is concentricallyinserted). The exhaust housing 43 is lined with a high-temperatureinsulation sleeve 44 that fills the space between the inner diameter ofthe exhaust housing 43 and the outer diameter of the heat exchanger 42.This insulation acts as a barrier between the heat exchanger 42 and thephysical structure of the exhaust housing, keeping the temperatures lowenough to allow it to be manufactured from regular mild steel, forexample.

An insulation sleeve 44 locks in the helical annulus 42E (FIG. 4) of theoutside surface created by the helical heat exchanger air channels 42D.The combustion air passes axially and helically at approximately 0.8inches per revolution for 7 inches, for example, through the roundedrectangular channels. After passing through approximately ninerevolutions, for example, all air combines at 42B (FIG. 2) in thetransition between the heat exchanger 42 and combustion tube 47, whichmay be approximately 3.25 inches long, for example.

Heat exchanger 42 and combustion tube 47 may be connected by aspecialized silicon carbide thread 47A (FIG. 2). Heat exchanger 42 mayhave a special female thread, while combustion tube 47 may have specialmale thread. This connection allows for the heat exchanger to be astandard length, and the combustion tube to be adjusted to suit based onthe furnace application. Once the combustion air has entered thecombustion tube 47, it enters another helical annulus created by theouter diameter 50 (FIG. 2) of the gas tube, the inner diameter of thecombustion tube 47 (FIG. 2), and the inner diameter 47B of a spiralfinned pathway that runs axially downstream at a rate of approximately1.67 inches per revolution, for example, and ends at the combustionnozzle 32. As the combustion air moves axially and radially for 5revolutions, for example, the air is brought across the staging gasnozzle 51, where natural gas is injected into the combustion air byeight small holes 51A (FIG. 5) positioned about 45 degrees apart, forexample, circumferentially. What is then a gas/air mixture, continues totravel axially and radially at 1.67 inches per revolution, for example,for approximately two full revolutions before moving into the combustionnozzle 32. Concurrently, a staged gas extension tube 51D (FIG. 5)injects gas downstream ahead of the centrifugal air and gas mixture intothe combustion nozzle 32 and into the inner radiant tube 27 tointentionally stage combustion. The cross sectional area ratio of theradial holes to axial hole ranges from 1:1 to 10:1 is such that theamount of gas that can exit radially is between 50% and 90% of the totalgas.

The gas/air mixture then enters the outlet end of the combustion tube 47and is sent through a conically shaped reducer 32 designed to increasethe velocity of the flame as aperture 32A directs the flame toward theinner radiant tube 27 (FIG. 1). Combustion is completed inside the innerradiant tube 27 and the hot products of the combustion are passed downthe tube toward the downstream end of outer tube 24, where the hot gasesare turned 180 degrees and forced to flow in the reverse directiontoward the first end of the inner radiant tube 31 through the annulargap between outer radiant tube 23 and inner radiant tube 27. As theproducts of combustion near the first end of inner radiant tube 31, thehigh velocity flame created by the conically shaped reducer entrain someof the gases causing recirculation back into the ongoing combustion.

Both of the helical inserts in the heat exchanger 42 and combustion tube47 may be constructed of silicon carbide. The helical fluid channelingdesign increases the conductive heat transfer surface area that is theouter ceramic walls of both heat exchanging surfaces. A silicon carbidecomposition is advantageous in that both components experience lessthermal expansion when subjected to significant temperature changes thanwould be seen if produced out of another material. This also enhancesthe ability of the helical heat exchangers to match and couple with therest of the burner system, reducing thermally-induced stresses that canbe associated with inter-component couplings during high-temperatureoperating conditions.

When the products of heat generation exit the annulus, a portion of theproducts of heat generation are recirculated, while a significantportion enters an annular channel created by the inner diameter 15(FIG. 1) of the outer radiant tube 23, the outer diameter 16 of thecombustion tube 47, and a spiral finned (e.g., 1.67 inches perrevolution) pathway running the length of the combustion tube 47 towardthe exhaust housing 43. The cross section of this spiral finned pathway47B is shown in FIG. 2 as an extension of the fins located on the insideof the combustion tube 47; therefore, setting the fluid path on bothsides of the combustion tube in sequence for conductive heat transferthrough the ceramic wall separating the fluids. The increased heattransfer surface area created by the inner and outer finned combustiontube 47 reduces the exhaust temperature of the flue gas enough such thatthe highly effective heat exchanger 42 may be mounted into the externalportion (exhaust housing 44) of the single-ended radiant tube burnersystem, keeping the temperatures low enough to allow it to bemanufactured from regular mild steel, for example.

As the products of combustion exit the first recuperative sectioncreated by the combustion tube 47, the products transition into theexhaust housing 43 where the products enter the helical gap 42E (FIG. 4)formed from the heat exchanger 42 helical combustion air channels 42Dand the insulation sleeve 44 (FIG. 2). The combustion gases pass throughthe approximate 0.8 inches per revolution annular gap, for example, forthe entire axial length of the heat exchanger and finally exit theburner system via the exhaust housing outlet 33 (FIG. 1). Thetemperature of the heat exchanging fluid at the exhaust housing outlet33 is such that the heat that would otherwise be lost to the atmospherehas been transferred to the combustion air, heating it to temperaturesbetween 1050° F. and 1250° F. and drastically improved the efficiency ofthe self-recuperating single-ended radiant tube burner system.

Example features of the above include a combustion tube that ishelically finned on inside and outside, forming a spiral finned pathway(which may run 1.67 inches per revolution, for example) setting thefluid path of both the combustion air (inside) and hot products ofcombustion (outside) in sequence for conductive heat transfer through aceramic wall separating the fluids. An increased heat transfer surfacearea created by the inner and outer finned combustion tube reduces theexhaust temperature of the flue gas enough such that the highlyeffective heat exchanger may be mounted into the external portion(exhaust housing) of the single-ended recuperative (SER) burner, keepingthe temperatures low enough to allow the exhaust housing to bemanufactured from regular mild steel, for example. Inner helical finscan also provide improved mixing characteristics as natural gas isdispersed into the already swirling combustion air by the gas nozzle,which is strategically placed downstream from the combustion nozzle. Theimproved mixing leads to a decrease in combustion losses as the gas/airmixture is accelerated through the conically shaped reducer (combustionnozzle) and the flame is ignited.

Another example feature includes the particular selection and assemblyof the combustion tube and heat exchanger. Silicon carbide compositionis advantageous in that both the combustion tube and heat exchangerexperience less thermal expansion when subjected to significanttemperature changes than would be seen if produced out of anothermaterial. This also enhances the ability of the helical heat exchangersto match and couple with the rest of the burner system, reducingthermally-induced stresses that can be associated with inter-componentcouplings during high-temperature operating conditions. The combustiontube and heat exchanger can be connected by a specialized siliconcarbide thread, where the heat exchanger has a special female thread andthe combustion tube has a corresponding male thread. This threadingallows for the heat exchanger to be a standard length and the combustiontube length to be conditioned for the specific furnace application.

Another example feature includes the heat exchanger being used for aunique channel orientation. Combustion air can be passed into individualports (e.g., six ports) in the introductory section that forms intorounded rectangular channels that pass the combustion air axially andhelically at approximately 0.8 inches per revolution, for example. Thecombination of short period and helical structure drastically increasethe heat transfer surface area and allow maximum heat transfer betweenthe incoming combustion air and outgoing products of combustion. Theheat exchanger can operate without the need to specify an oversizedblower or expanded method to produce forced air at an increased rate toovercome pressure drop caused by channel design.

Another example feature includes a silicon carbide axial tube throughthe gas tube nozzle. Silicon carbide provides enhanced exposure to thehigh-temperature environment with minimal thermal expansion, allowingsteady and consistent dispersion of natural gas. Radial holes in thenozzle inject gas into the spiral flow of pre-heated combustion airincreasing mixing characteristics leading to decreased combustionlosses. Axial holes allow for a spark and flame rod to be internal tothe gas tube and inserted to the point of combustion. An axial tubethrough the gas tube nozzle allows gas to flow axially downstream aheadof the centrifugal air and gas mixture into the combustion nozzle andinto the inner radiant tube to intentionally stage combustion.

The disclosed example embodiments provide advantages over prior systems,such as increased efficiency, a more customizable length for thecombustion tube, uniformity with hot spot over average (HSOA) being lessthan 50 degrees Fahrenheit (which provides more even heating to the loadand longer tube life when using an alloy outer tube), NOx emissions lessthan 240 ppm and CO emissions less than 10 ppm at 3% oxygen across allfire rates, and the option of an all-ceramic design (e.g., gas nozzle,inner tube, outer tube, heat exchanger, and combustion tube) that allowsfor high-temperature application and reduced maintenance cycles overexisting alloy and ceramic single-ended recuperative (SER) burners.

Operation of Staged Recirculating Burner and Radiant Tube Burner System

As disclosed above, an example particular embodiment may include, asshown in FIG. 2, a gas tube 37, exhaust housing 43, preheat flow reducer42A, exhaust insulating sleeve 44, threaded combustion tube joint 47A,inner and outer finned combustion tube 47, staging gas nozzle 51,combustion nozzle 32, centering spacer 29, air/gas inlet housing 34, airinlet 36, gas inlet 35, gas staging tube 10, and heat exchanger 42. Sucha staged recirculating burner may operate in a radiant tube combustionsystem that may include, as shown in FIG. 1, an inner furnace wall 20,furnace refractory 21, outer refractory wall/shell 22, outer radianttube 23, outer radiant tube cap and support 24, refractory furnaceopening 25, outer radiant tube flange 26, support flange 14, innerrecirculating tube 27, flame rod 18, and igniter 19.

As an example of operation, a gaseous fuel enters the gas inlet 35 andof the air/gas inlet housing 34 and air enters the air inlet 36 of theair/gas inlet housing 34 in an air-to-gas ratio of approximately between5:1 and 15:1, for example, which is sufficient to, when ignited, producea flame and products of combustion. The gaseous fuel travels down thegas tube 37 where it enters the staging gas nozzle 51, which can includeboth radial exit holes and an axial gas staging tube 10. The crosssectional area ratio of the radial holes to axial tube may range from1:1 to 10:1, for example, such that the amount of gas that can exitradially is between 50% and 90% of the total gas. Simultaneously withthe gaseous fuel, air enters the fluid inlet of the heat exchanger 42and the inner spiral channel of the heat exchanger 42, which may have asubstantially rectangular cross-section. The air receives energy fromthe outer wall of the spiral channel and is preheated to a temperaturegreater than 400 degrees Celsius before it exits the spiral channel ofthe heat exchanger 42 as preheated air and then flows into the preheatflow reducer 42B. The outer wall of the spiral channel receives energyfrom products of heat generation that flows through the surroundingfluid path that the outer spiral channel forms by the outer wall with asubstantially rectangular cross-section. The products of heat generationare cooled as the energy is transferred to the outer wall and further tothe air flowing through the heat exchanger 42. The products of heatgeneration exit through the exhaust housing 43. The exhaust housing 43contains an exhaust insulating sleeve 44 that minimizes the heat lost tothe atmosphere such that the maximum amount of heat can be transferredto the outer spiral wall and, thus, the air.

The preheated air enters the preheat flow reducer attached to the innerand outer finned combustion tube 47, which itself may be attached to thepreheat flow reducer by a threaded ceramic combustion tube joint 47A.The preheated air is further heated to a highly preheated airtemperature that exceeds 500 degrees Celsius in the inner and outerfinned combustion tube 47, which contains one or more spiral fins, bythe products of heat generation flowing on the outside of the inner andouter finned combustion tube 47. The products of heat generation arecooled by the inner and outer finned combustion tube to a point wherethe mounting of the outer radiant tube 23 and outer radiant tube flange26 can be mounted between the step flange 14 and the exhaust housing 43flange without the use of exotic high-temperature materials.

The highly preheated air exits the fins of the inner and outer finnedcombustion tube 47 in a spiral flow path where a staging gas nozzle 51is positioned to inject gas radially into the spiral flow of highlypreheated air. The position of the staging gas nozzle 51 and its radialholes is such that the mixture of air and gas is properly mixed to forma mixture that can be ignited by tip of the igniter 19, and that flowsinto and further combusts in the combustion nozzle 32 attached to theinner and outer finned combustion tube 47 by a high-temperature ceramicthreaded connection, for example. Not all of the gas is injected throughthe radial holes of the staging gas nozzle 51. The mixture that isignited is substantially lean to suppress the temperature of products ofheat generation, which suppresses the formation of oxides of nitrogen.The products of heat generation exit the combustion nozzle 32 at avelocity sufficient to entrain products of heat generation flowingthrough an annulus formed by the inner recirculating tube 27 and theouter radiant tube 23 and further through the opening formed between thecombustion nozzle exit 32A and the inside of the inner recirculatingtube 27. The products of heat generation are at a sufficiently lowtemperature that the products of heat generation exiting the combustionnozzle are diluted sufficiently to further reduce the formation ofoxides of nitrogen before the products of heat generation are fullycombusted inside the inner recirculating tube 27 by exhaust gasrecirculation.

The final amount of gas is injected into the partially combustedproducts of heat generation by an axial tube that may extend from thestaging gas nozzle 51 and into the combustion nozzle 32. The gas iscombusted fully before exiting the end of the inner recirculating tube27. The combination of lean combustion, recirculating of products ofheat generation, and gas staging of the products of combustion issufficient to suppress the formation of oxides of nitrogen, minimize thetemperature of combustion for suppression of the formation of oxides ofnitrogen, and improve the temperature uniformity of heat released fromthe outer radiant tube 23.

The products of heat generation may be directed between the annulusformed by the outer radiant tube 23 and the inner recirculating tube 27by a centering spacer (turning vane) that may include at least twouniform fins and promotes the reversal of flow from the products of heatgeneration into the formed annulus. As the products of heat generationflow between the formed annulus, a substantial amount of energy istransferred to the wall of the outer radiant tube 23 by both convectionand radiation heat transfer. Energy is transferred through the wall ofthe outer radiant tube 23 by conduction. A substantial amount of energyis transferred from the outer radiant tube 23 to the inner furnace wall20 by radiation heat transfer. When the products of heat generation exitthe annulus, a portion of the products of heat generation isrecirculated, while a significant portion enters the outer fins of theinner and outer finned combustion tube 47. The products of heatgeneration are cooled, as described above, by flowing over the inner andouter finned combustion tube 47 and heat exchanger 42 before exiting thesystem at the exhaust housing 33 exit.

While example embodiments have been particularly shown and describedabove, it will be understood by those skilled in the art that variouschanges in form and details may be made therein without departing fromthe scope of the invention. For example, the outer tube 23 may befinned, lobed, and/or twisted for improved heat transfer. The inner tube27 may be finned, lobed, and/or segmented for improved heat transferuniformity, combustion, and recirculation. The combustion nozzle 32, mayinclude single or multiple nozzles, which are not necessarily round, andmay include hole extensions for air staging. The gas nozzle 10 mayinclude radial holes, axial holes, tangential holes, and or angledholes, which are not necessarily round, and may include hole extensionsfor gas staging. Holes can, for example, be round, oval, square, slots,or porous. The combustion tube 17 may be differently finned, lobed,and/or twisted for improved heat transfer. The turning vane 29 can bespiral or U-shaped with an inlet point to separate the flow. The innertube 27 and outer tube 23 can use staged spiraling (rifling) or stagedfins in order to reduce variation in the hot spot above average (HSOA)and reduce the HSOA value. As an example, the first one-third of thetube may be smooth and the last two-thirds finned. The outer tube 23 maybe an alloy tube with a de-tuned silicon carbide air heater, which wouldbe a low air pressure, reduced efficiency system that would allow for ahigher pressure, higher efficiency, air heater to be installed. The heatexchanger 42 may be finned, lobed, and/or segmented for improved heattransfer uniformity, combustion. The gas nozzle 10 could be extended orretracted, of variable length, in combination with changes in shape anddiameter of the conical reducer 32 and aperture 32A to change theemissions and thermal characteristics of the radiant tube heatingsystem.

What is claimed is:
 1. A staged recirculating burner, comprising: acombustion tube including inside and outside helical fins formingopposing spiral pathways for combustion gases and products ofcombustion; a combustion nozzle coupled to the combustion tube; a gastube running axially into the combustion tube; and a heat exchangercoupled to the combustion tube to heat the combustion gases provided tothe combustion tube using the products of combustion from the combustiontube.
 2. The staged recirculating burner of claim 1, wherein the gastube runs axially through a central bore of the heat exchanger and intothe combustion tube.
 3. The staged recirculating burner of claim 1,wherein the heat exchanger is coupled to the combustion tube.
 4. Thestaged recirculating burner of claim 1, wherein the heat exchanger isconnected to the combustion tube via a ceramic thread.
 5. The stagedrecirculating burner of claim 4, wherein the ceramic thread comprisesSiC.
 6. The staged recirculating burner of claim 1, wherein thedirection of flow of the combustion gases and the direction of flow ofthe products of combustion are opposite, and the combustion tubeincludes a ceramic wall separating the flow of the combustion gases andthe flow of the products of combustion.
 7. The staged recirculatingburner as in claim 1, wherein the combustion gas nozzle is conicallyshaped combustion nozzle.
 8. The staged recirculating burner as in claim1, comprising a staging gas nozzle coupled to the gas tube, the staginggas nozzle including radial exit holes into the combustion tube and anaxial gas staging tube extending into the combustion nozzle to stage. 9.The staged recirculating burner as in claim 8, wherein the staging gasnozzle is configured to inject only a portion of the gas through theradial holes to create a gas mixture that is substantially lean tosuppress a temperature of products of combustion, and is configured toinject the reminder of the gas through the axial gas staging tube. 10.The staged recirculating burner as in claim 1, wherein the heatexchanger comprises a plurality of helical flow paths configured to heatthe combustion gas, wherein preheated combustion gas coming from theplurality of helical flow paths combines in a transition between theheat exchanger and the combustion tube before entering the combustiontube.
 11. A staged recirculating burner, comprising: a combustion tubeincluding inside and outside helical fins forming opposing spiralpathways for combustion gases and products of combustion; a heatexchanger coupled to the combustion tube configured to provide preheatedcombustion gases to the combustion tube; and a gas tube running axiallythrough a central bore of the heat exchanger and into the combustiontube.
 12. The staged recirculating burner of claim 11, wherein the heatexchanger comprises a plurality of helical combustion air channelsextending around the central bore in a length direction of the heatexchanger, wherein each helical combustion air channel defines a flowpathway for combustion air to flow through the helical combustion airchannel.
 13. The staged recirculating burner of claim 12, furthercomprising a preheat flow reducer disposed at a downstream end of theheat exchanger and configured to combine preheated combustion air fromthe plurality of helical combustion air channels into a single flow intoa combustion tube.
 14. A radiant tube burner system, comprising: thestaged recirculating burner of claim 11; an outer radiant tube coupledto the burner; an inner recirculating tube located concentrically insidethe outer radiant tube, the outer radiant tube and the innerrecirculating tube forming an annulus therebetween; and a turning vanespacer located inside the outer radiant tube and positioned between thedistal end of the inner recirculating tube and the distal end of theouter radiant tube to cause products of combustion to flow through theannulus between the outer radiant tube and the inner recirculating tube.15. The radiant tube burner system of claim 14, wherein the outerradiant tube and the inner recirculating tube comprise a ceramicmaterial.
 16. The staged recirculating burner of claim 11, comprising: acombustion nozzle coupled to the combustion tube; a gas tube runningaxially into the combustion tube; and a staging gas nozzle coupled tothe gas tube, the staging gas nozzle including radial exit holes intothe combustion tube and an axial gas staging tube extending into thecombustion nozzle to stage combustion.
 17. The staged recirculatingburner of claim 11, wherein the heat exchanger insert is adapted so thatthe combustion air flowing through the plurality of combustion airchannels is preheated to a temperature greater than 400° C. by energyfrom the combustion product passing through the surrounding fluid pathwhen the combustion air exits the helical combustion air the channels.18. The staged recirculating burner of claim 11, wherein the heatexchanger comprises a helical annulus including a plurality of helicalcombustion air channels extending around the central bore in a lengthdirection of the body, wherein each helical combustion air channeldefines a flow pathway for combustion air to flow through the helicalcombustion air channel.
 19. The staged recirculating burner of claim 18,wherein the heat exchanger comprises an introductory port section havingan individual port for each of the plurality of helical combustion airchannels.
 20. The staged recirculating burner of claim 11, wherein theheat exchanger comprises a ceramic threaded portion adapted to connectto a complementary threaded portion of the combustion tube.